Clinical application of selective spinal analgesia is limited by side effects, primarily respiratory, depression with opioids and hypotension with alpha2-adrenergic agonists. This collaborative application examines a new approach to selective spinal analgesia to minimize or avoid these side effects. Alpha2-Adrenergic agonists may produce analgesia by activating spinal cholinergic neurons. Intraspinal clonidine causes acetylcholine (ACh) release into cerebrospinal fluid (CSF) in sheep and humans, and clonidine-induced analgesia is inhibited by spinal atropine and enhanced by spinal neostigmine. We will test in humans whether spinal neostigmine enhances epidural clonidine analgesia by increasing clonidine-induced ACh in CSF, while at the same time diminishing clonidine's hypotensive actions. Using more precise agonists and antagonists in rats, we will test whether neostigmine interacts with alpha2-agonists in an additive or synergistic manner, and whether neostigmine increases the relative efficacy of alpha2-agonists as a function of their relative efficacy alone. Systemic opioids activate descending inhibition mediated in large part by spinal cholinergic neurons. We will test in humans whether spinal neostigmine enhances iv alfentanil induced analgesia by increasing alfentanil-induced ACh release in CSF. Physiologic studies suggest that spinal neostigmine's effects are prolonged and could provide profound analgesia postoperatively by simple iv techniques after a single spinal injection. These questions can be tested rapidly, safely, and precisely by combining the expertise of these two laboratories which are capable of completing preclinical toxicity testing, IND application, and performing phased clinical studies complemented by detailed pharmacologic studies in animals.